A belt-type printing machine of the kind mentioned above is known from EP 0 018 147 B1. The continuous web to be printed is guided over idler rolls supported on the frame of the machine and over an impression cylinder, one for each printing station. It is the impression cylinder, which is displaceably mounted on the frame to start printing, which is disadvantageous for the tensioning relations of the web, but is advantageous for the possibility to mount the unit of the plate cylinder and the sprocket wheels in bearings being fixedly positioned on the frame of the machine. Opposite the impression cylinder of this unit is a plate cylinder and two sprocket wheels, with their axes coaxial with the plate cylinder, mounted in fixed bearings. The plate cylinder may rotate independently of the two sprocket wheels, but the two sprocket wheels are mechanically connected to each other and rotate together. An endless belt extends around this unit, which is provided with at least one, but often with a plurality of, flexible printing plates. A first drive with positive engagement is allocated to the belt, which acts upon the two sprocket wheels to the belt. The sprocket wheels are connected in rotation to each other and the pins of the sprocket wheels engage into the perforations provided in the belt to maintain accurate registration during printing. In the travel of the flexible printing plates on the belt through the nip between the plate cylinder and the impression cylinder, and the web, respectively, a squeezing and a bulge occurs in the flexible material of the printing plates and thus a high drag or resistance to movement of the printing belt results directed opposite to the running direction. This resistance force, which may be called a force against the change of the shape of the printing plates, only exists in the printing process, but not when there is a distance between the web and the printing plates within the idler running of the machine, when printing is not performed. This resistance varies dependent on the shape of the printing plates running through the nip. This resistance force during the printing process may be higher, or become higher than the drive forces, which may be transmitted by the sprocket wheels to the belt and thus higher than the maximum force allowable between the pins of the sprocket wheels and the perforations of the belt. In such a case, one perforation will jump from one pin of the sprocket wheel to the next, and registration will be lost and the printed web cannot be used to obtain a proper printing result. To solve this problem the known belt-type printing machine provides an additional drive, a frictional drive in addition to the drive torque with positive engagement transmitted by the sprocket wheels to the belt. This additional frictional drive acts upon the plate cylinder to the belt. The plate cylinder may rotate independant from the sprocket wheels. The resistance force directed opposite to the running direction only occurs during the printing process at a printing plate, when the printing plate passes the nip. Very often there is not only one printing plate on the belt, but a plurality of plates with distances between each other. Thus during the running of the belt times arise in which the resistance force disappears. Even when a printing plate on the belt is passing the nip the resistance force varies. The amount of the resistance force depends on the amount of the area of the printing plate contacting the nip. To accomodate the varying resistance force the known belt-type printing machine is provided with a very complicated controlling means. This controlling means comprises sensors for continuously detecting a signal being proportional to the resistance force and occurring in the drive of the sprocket wheels. The generated data of this signal must be transmitted via a slip ring contact. The sensors use strain gauges to measure forces and torques. In addition, the controlling means comprises a clutch and a brake, which belong to a controlling loop and by which the additional drive to the plate cylinder is controlled. The additional drive is split from the main drive of the printing machine. In this known belt-type printing machine the sprocket wheels and the plate cylinder are used for driving purposes. The drive commonly acts upon the sprocket wheels to the belt. The additional drive acts upon the plate cylinder to the belt. It is disadvantageous and not easy to transmitt the drive on the one hand, and the additional drive on the other hand, to the sprocket wheels and the plate cylinder when they are in a coaxial and restricted arrangement. In addition, the sensitive parts, and especially the sensors (load cells) of the controlling means, are located in an area of the belt-type printing machine in which they are exposed to cleaning water and other detergents, and to colors inks also. The mechanical fixing of the two sprocket wheels to each other via the common drive is subject to wear. This is a negative for proper registration.
From EP 0 308 367 A1 a belt-type printing machine is known, in which the impression cylinder is mounted in fixed position in the frame of the machine. Thus there is the advantage that the tensioning conditions of the web to be printed do not vary when the belt with the printing plates is brought into contact with the web. But the plate cylinder must be mounted displaceably in the frame of the machine. The bearings of the plate cylinder have to be mounted displaceably in at least two directions, and the bearings of the tensioning roll in at least one direction. One drive for the tensioning roll and another drive for the plate cylinder has to be provided. It is not described whether and how the two drives are adapted to each other. In addition, a main drive is provided for driving the impression cylinder. DE 41 00 871 A1 shows a belt-type printing machine having an impression cylinder, which is mounted in fixed position on the frame of the machine. Complicated controlling means are avoided and the drive acting on the sprocket wheels is adapted to an additional drive to the belt. The additional drive acts on the tensioning roll to the belt. The plate cylinder is an idler roller having no drive. A controlling device is provided for the additional drive to transmitt an additional force to the belt in such a manner so that this force, one the one hand, during printing is higher than the difference between the resistance force directed opposite to the running direction when a printing plate runs through the nip between the web and the plate cylinder and the maximum force transmitted by the sprocket wheels to the belt, and on the other hand, so that this force during non-printing must be lower than the maximum force transmitted by the sprocket wheels to the belt.
In a number of cases the printing plate on the belt is not located symmetrically with respect to the vertical main plane extending in the direction of the printing machine. Thus, different loads result for the perforations on one side of the machine, compared to the perforations on the other side of the machine. The sprocket wheel on the one side of the machine transmits a greater amount of the drive torque than the sprocket wheel on the other side of the machine to comply with the different parts of the resistance force at the left and the right side of the machine. The fact that the belt containing the printing plates is a quasielastic body results in the disadvantage that unequal loads and unequal drives lead to different angular positions of the belt at the right side and at the left side of the machine. Thus registration at the right side is different from registration on the left side. Consequently different conditions (pressure on the face of a hole) in the area of the right and left perforations may occur. Unequal wear is also generated, making the problems even greater. This may lead to a situation in which the perforations of the belt on its more loaded side will jump against its sprocket wheel so that the printing result cannot be used, and is rejected.